1. Field of the Invention
The invention relates to a method of fabricating a liquid crystal display device, and more particularly to such a method including the step of forming an electrically conductive transparent film in a predetermined pattern on an interlayer insulating film comprised of an electrically insulating organic film.
2. Description of the Related Art
A liquid crystal display device is grouped into a light-transmission type one, a light-reflection type one and a combination type one in dependence on a light source.
A light-transmission type liquid crystal display device is designed to include a back-light device as a light source for displaying images. A light-reflection type liquid crystal display device is designed to include a light-reflector, at which incident light is reflected towards a viewer. Hence, a light-reflection type liquid crystal display device is not necessary to have a back-light device as a light source unlike a light-transmission type liquid crystal display device. A combination type liquid crystal display device is designed to have the same structure as that of a light-transmission type liquid crystal display device for a half of a display area and further have the same structure as that of a light-reflection type liquid crystal display device for the rest of a display area.
Among the above-mentioned three type liquid crystal display devices, a light-transmission liquid crystal display device is usually designed to include a thin film transistor (TFT) or a metal-insulator-metal (MIM) as a switching device for driving and controlling a pixel electrode. A pixel electrode is usually comprised of an electrically conductive transparent film such as an indium-tin oxide (ITO) film for ensuring high light-transmissivity and low resistance.
Many methods have been suggested for patterning indium-tin oxide (ITO) into a pixel electrode.
For instance, Japanese Patent Application Publication No. 6-88973 has suggested a method of patterning ITO into a pixel electrode, including the steps of depositing indium-tin oxide on an inorganic interlay insulating film by sputtering at 0 to 100 degrees centigrade, patterning the deposited indium-tin oxide into a pixel electrode, and annealing the pixel electrode at 200 to 400 degrees centigrade, preferably at 230 to 380 degrees centigrade, in hydrogen atmosphere. It is said in the Publication that annealing for reducing dangling bond in a semiconductor layer of a thin film transistor and further for enhancing electrical characteristics and annealing for light-transmissivity of ITO and further for reducing electrical resistivity of ITO can be carried out as single annealing.
Japanese Patent Application Publication No. 9-258247 has suggested a method of patterning ITO into a pixel electrode on an electrically insulating organic film such as an acrylic resin film, including the steps of depositing indium-tin oxide on an electrically insulating organic film composed of acrylic resin by sputtering at about 230 degrees centigrade, annealing the indium-tin oxide immediately after the deposition of the indium-tin oxide at a temperature equal to or greater than 100 degrees centigrade but equal to or smaller than a temperature at which the indium-tin oxide was deposited, and patterning the indium-tin oxide into a pixel electrode. It is said in the Publication that it is possible to reduce a line-width shift in the step of patterning ITO by carrying out annealing after deposition of ITO.
If an interlayer insulating film comprised of an electrically insulating organic film is annealed at 230 degrees centigrade or higher as suggested in the above-mentioned Japanese Patent Application Publication No. 6-88973, the electrically insulating organic film would be usually decomposed, resulting in reduction in light-transmissivity. Hence, annealing for reducing dangling bond in a semiconductor layer of a thin film transistor and enhancing electric characteristics of the same is usually carried out prior to formation of an electrically insulating organic film independently of annealing to be carried out after deposition of indium-tin oxide.
Japanese Patent Application Publication No. 2001-343901 has suggested a method of patterning indium-tin oxide both on an electrically insulating organic film such as an acrylic resin film and an electrically insulating inorganic film such as a SiN film. The suggested method includes the steps of depositing indium-tin oxide on both on an electrically insulating organic film and an electrically insulating inorganic film, annealing the indium-tin oxide at 150 to 220 degrees centigrade, preferably at 200 to 220 degrees centigrade, and patterning the indium-tin oxide to a pixel electrode subsequently to the annealing step.
Japanese Patent Application Publications Nos. 2001-345023 and 2001-345024 suggest a method of patterning ITO into a pixel electrode, including the steps of depositing indium-tin oxide on an electrically insulating organic film, applying O2, Ar or CF4 plasma to the indium-tin oxide such that the indium-tin oxide has a crystal diameter in the range of 20 nm to 50 nm, and patterning the indium-tin oxide into a pixel electrode. It is said in the Publications that by carrying out annealing after patterning indium-tin oxide into a pixel electrode or applying plasma to indium-tin oxide before patterning the indium-tin oxide into a pixel electrode, indium-tin oxide deposited on an electrically insulating organic film and indium-tin oxide deposited on an electrically insulating inorganic film would have etching rates almost equal to each other, resulting in that a line-width shift can be reduced in the step of patterning ITO into a pixel electrode. It is further said in Japanese Patent Application Publication No. 2001-345023 that the indium-tin oxide may be annealed at 150 to 220 degrees centigrade after the step of patterning the indium-tin oxide into a pixel electrode.
Japanese Patent Application Publication No. 10-161158 has suggested a method of patterning indium-tin oxide into a pixel electrode on an electrically insulating organic film such as a poly-imide resin film or an acrylic resin film. The suggested method includes the step of roughing a surface of an electrically insulating organic film by sputtering etching, dry etching or irradiation of ultra-violet rays before carrying out a step of patterning ITO into a pixel electrode. By carrying out the step of roughing the surface, ITO and the electrically insulating organic film would make contact with each other through a large area, which ensures close contact between ITO and the electrically insulating organic film. As a result, it would be possible to accurately pattern ITO into a pixel electrode. The step of roughing the surface may be carried out prior to or subsequently to formation of a contact hole.
If indium-tin oxide is patterned into a pixel electrode on an electrically insulating organic film such as a acrylic resin film in accordance with the method suggested in Japanese Patent Application Publication No. 6-88973, the electrically insulating organic film would be decomposed, resulting in that the resultant pixel electrode would be colored, and hence, light-transmissivity would be reduced, because a temperature at which the indium-tin oxide is annealed is usually higher than a temperature at which an electrically insulating organic film is not decomposed. If an electrically insulating organic film is annealed at 200 to 300 degrees centigrade, it would be possible to prevent the electrically insulating organic film from being decomposed, but a range of 200 to 300 degrees centigrade is too low to reduce dangling bond in a semiconductor layer of a thin film transistor. Accordingly, it is impossible to design the method to include only one annealing step.
In accordance with the method suggested in Japanese Patent Application Publication No. 9-258247, an indium-tin oxide film is reformed by gas discharged from an electrically insulating organic film during patterning indium-tin oxide into a pixel electrode, resulting etching residue in the step of patterning indium-tin oxide into a pixel electrode. If indium-tin oxide is deposited at 100 degrees centigrade or lower, such etching residue can be suppressed to some degree, but it is not possible to completely suppress such etching residue. Since the method includes a step of carrying out annealing after deposition of indium-tin oxide, the method unavoidably carries out an annealing step twice.
In the methods suggested in the above-mentioned Japanese Patent Application Publications Nos. 2001-345023 and 345024, plasma is applied to indium-tin oxide before patterning the indium-tin oxide into a pixel electrode, namely, after formation of a contact hole throughout the electrically insulating organic film. According to the experiments having been conducted by the inventors, it was found out that contact resistance between the indium-tin oxide and an underlying metal film was increased in dependence on gas used for generating plasma. In particular, such contact resistance is significantly increased when fluorine-containing gas such as CF4 or helium (He) gas is used for generating plasma.
In the method suggested in the above-mentioned Japanese Patent Application Publication No. 10-161158, if ultra-violet rays are first irradiated to the electrically insulating organic film, the electrically insulating organic film would be decomposed, resulting in that the resultant pixel electrode would be colored.
In addition, if sputter etching or dry etching is to be carried out prior to formation of a contact hole, it would be possible to do so after baking an electrically insulating organic film, only if the electrically insulating organic film is not photo-sensitive. In contrast, if the electrically insulating organic film is photo-sensitive, it would be necessary to carry out sputter etching or dry etching after the electrically insulating organic film was formed. By carrying out sputter etching or dry etching in a condition in which the electrically insulating organic film has been already formed, an etching apparatus might be contaminated with the result of significant reduction in productivity, and a roughened surface of the electrically insulating organic film would be planarized during baking the electrically insulating organic film.
If sputter etching or dry etching is carried out after formation of a contact hole, there is caused a problem that the above-mentioned contact resistance is increased.
Increase in a contact resistance might cause horizontal cross-talk and/or non-uniformity of horizontal lines in a common-storage type twisted nematic liquid crystal display device or an in-plane switching type liquid crystal display device. Specifically, it is necessary in a common-storage type twisted nematic liquid crystal display device to bind common lines to one another for applying a common voltage to the common lines. However, if a thin film transistor is designed to have a structure in which common lines are bound to one another through an indium-tin oxide film formed on an interlayer insulating film, a contact resistance would be increased due to an electrically insulating organic film, and hence, the common lines would unavoidably have a high resistance.
Recently, an in-plane switching type liquid crystal display device is commercially available. An in-plane switching type liquid crystal display device includes a common electrode comprised of an electrically conductive transparent film and formed on an electrically insulating organic interlayer film above a signal line, and a pixel electrode comprised of the electrically conductive transparent film of which the common electrode is comprised, and formed in facing relation to the common electrode. Such an in-plane switching type liquid crystal display device has a high aperture ratio. For instance, an in-plane switching type liquid crystal display device is suggested in WO98/47044.
It is quite important for an in-plane switching type liquid crystal display device to accurately pattern an electrically conductive transparent film formed on an electrically insulating organic film, into a pixel electrode uniformly within a substrate. If process control for a step of patterning an electrically conductive transparent film into a pixel electrode is insufficient, there would be resulted in non-uniformity in displayed images. In addition, it is important for a common storage type in-plane switching type liquid crystal display device to reduce a contact resistance between an electrically conductive transparent film and an underlying metal film.
Thus, there is a need for a method of fabricating a liquid crystal display device which method is capable of enhancing accuracy with which an electrically conductive transparent film formed on an electrically insulating organic interlayer film is patterned into a pixel electrode, and accomplishing a low contact resistance between the electrically conductive transparent film and an underlying metal film.
Japanese Patent Application Publication No. 9-246210 has suggested a method of patterning an electrically conductive transparent film, including the steps of forming an electrically conductive transparent film on an organic or inorganic substrate or film, forming a mask on the electrically conductive transparent film in a predetermined pattern, implanting hydrogen ions into the electrically conductive transparent film through the mask, and etching for removal portions of the electrically conductive transparent film into which the hydrogen ions were implanted.
Japanese Patent Application Publication No. 9-293875 has suggested a method of fabricating a semiconductor device, including the steps of forming a light-permeable film on a monocrystal semiconductor substrate, fabricating a non-monocrystal semiconductor device on the light-permeable film, fabricating a monocrystal semiconductor device on the monocrystal semiconductor substrate, forming an electrically conductive amorphous film on the monocrystal semiconductor substrate, and re-crystallizing the electrically conductive amorphous film to make the film transparent.
Japanese Patent Application Publication No. 11-95239 has suggested a method of fabricating a liquid crystal display device, including the step of forming an indium-tin oxide film on an electrically insulating substrate by sputtering in which there is used water vapor having a partial pressure in the range of 0.002 Pa to 0.010 Pa.
Japanese Patent Application Publication No. 2002-296609 has suggested a method of fabricating a liquid crystal display device including a bus wire arranged on a substrate in a matrix, a switching device electrically connected to the bus wire, and a pixel electrode electrically connected to the bus wire with an interlayer insulating film being sandwiched therebetween, the method including the step of keeping the substrate at 100 to 170 degrees centigrade while an electrically conductive transparent film is formed on the interlayer insulating film.